Probability of Immobilization on Host Cell Surface Regulates Viral Infectivity

Abstract

The efficiency of a virus to establish its infection in host cells varies broadly among viruses. It remains unclear if there is a key step in this process that controls viral infectivity. To address this question, we use single-particle tracking and Brownian dynamics simulation to examine human immunodeficiency virus type 1 (HIV-1) infection in cell culture. We find that the frequency of viral-cell encounters is consistent with diffusion-limited interactions. However, even under the most favorable conditions, only 1% of the viruses can become immobilized on cell surface and subsequently enter the cell. This is a result of weak interaction between viral surface gp120 and CD4 receptor, which is insufficient to form a stable complex the majority of the time. We provide the first direct quantitation for efficiencies of these events relevant to measured HIV-1 infectivity and demonstrate that immobilization on host cell surface post-virion-diffusion is the key step in viral infection. Variation of its probability controls the efficiency of a virus to infect its host cells. These results explain the low infectivity of cell-free HIV-1 in vitro and offer a potential rationale for the pervasive high efficiency of cell-to-cell transmission of animal viruses.

FIG. 1.

Imaging and quantifying the dynamics of HIV-1 and TZM-bl interactions at short timescales. (a) High-speed imaging allows reconstruction of multiple tracks of HIV-1 virions from a 1000-frame video overlaid on a differential interference contrast image of the cells (boundaries outlined in red solid lines). The scale bar is 10 μm. (b) The rates of collision measured for HIV-1 virions under various conditions and for reference fluorescent beads with a mean diameter of 140 nm.

FIG. 2.

Virion immobilization and the dynamics of transient virion-cell encounters. (a) Probability of immobilization determined for various particles and conditions. (b),(c) Histograms for number of touching events per trajectory and touching times determined for HIV_0.0 (green), HIV_0.2 (red), and HIV_1.0 virions (blue) in the absence of DEAE-D. Histograms in (c) were fit by single-exponential functions for HIV_0.0 (green dash-dotted line), HIV_0.2 (red dashed line), and HIV_1.0 virions (blue solid line). The number of tracks for each virion population in (b) and (c) were 409, 512, and 359, respectively. (d) Rate of immobilization determined for various conditions. In both (a) and (d), conditions 0, 1, 2, 3, and beads are for TZM-bl cells.

FIG. 3.

Time-lapse imaging of HIV_1.0 and TZM-bl interactions and the dynamics of internalized virions. (a) While many virions transiently interact at the cell surface before diffusing away, virion immobilization and internalization can be clearly identified (tracks 1–8). In particular, track 7 can undergo extensive intracellular transport. The scale bar is 10 μm. (b) Corresponding boxcar-filtered fluorescence intensities using a 10-s wide window for the labeled tracks in (a). (c) Distributions of measured intensities for freely diffusing (blue bars) and internalized HIV_1.0 virions (red bars) at focus.